1. Field of the Invention
This invention relates to a process for making a copolymer of maleic anhydride and a C.sub.1 -C.sub.4 alkyl vinyl ether, and, more particularly, to a solvent-free process for making such copolymer product in the form of free-flowing, fine white powders having a predetermined, substantially reduced specific viscosity, SV which is characterized by addition of a small amount of an active chain transfer agent to the reaction mixture, which agent can be easily volatilized without leaving substantially any residue, odor or taste in the product.
2. Description of the Prior Art
M. Tazi et al, in U.S. Pat. No. 5,003,014, described a process for making copolymers of maleic anhydride and a C.sub.1 -C.sub.4 alkyl vinyl ether having specific viscosities extending over a wide range by addition of large, solvent quantities of toluene as chain transfer agent in the polymerization. However, toluene is a relatively inactive chain transfer agent for this polymerization. Accordingly, such solvent amounts of toluene were required to effect any reduction in the specific viscosity of the copolymer product. Furthermore, toluene was difficult to remove during the process steps of venting of excess alkyl vinyl ether and drying of the polymer, resulting in a toluene residue in the product, and some objectionable odor and taste.
Shimizu, H., in Jap. Kokai pat. Appln. No. Hei 6 [1994]-569; published Mar. 1, 1994, described a process of making very low molecular weight copolymers of maleic anhydride and alkyl vinyl ethers by including large amounts of higher alkyl aldehydes as a solvent in the system. Such aldehydes, however, are non-volatile and remain in the product giving it a particularly unpleasant odor.
Accordingly, it is an object of the present invention to provide a solvent-free process for making copolymers of maleic anhydride and a C.sub.1 -C.sub.4 alkyl vinyl ether in the presence of an active chain transfer agent which can be included in small amounts in the polymerization mixture to effect a substantial reduction in the specific viscosity of the copolymer product, and which can be easily volatilized during removal of excess alkyl vinyl ether without degrading the copolymer and without leaving substantially any residue, odor or taste in the product.
A specific object is to provide copolymers of maleic anhydride and methyl vinyl ether which have a specific viscosity SV of about 0.5 to 5, preferably 1.0 to 4, by including about 0.01 to 1.9%, preferably 0.02 to 1.0, by weight of the reaction mixture, of acetaldehyde therein.